Description

CREATE AGGREGATE defines a new
aggregate function. Some basic and commonly-used aggregate
functions are included with the distribution; they are documented
in Section 9.15. If one
defines new types or needs an aggregate function not already
provided, then CREATE AGGREGATE can be
used to provide the desired features.

If a schema name is given (for example, CREATE AGGREGATE myschema.myagg ...) then the
aggregate function is created in the specified schema. Otherwise
it is created in the current schema.

An aggregate function is identified by its name and input data
type(s). Two aggregates in the same schema can have the same name
if they operate on different input types. The name and input data
type(s) of an aggregate must also be distinct from the name and
input data type(s) of every ordinary function in the same
schema.

An aggregate function is made from one or two ordinary
functions: a state transition function sfunc, and an optional final calculation
function ffunc. These are used as
follows:

PostgreSQL creates a
temporary variable of data type stype to hold the current internal state of
the aggregate. At each input row, the aggregate argument value(s)
are calculated and the state transition function is invoked with
the current state value and the new argument value(s) to
calculate a new internal state value. After all the rows have
been processed, the final function is invoked once to calculate
the aggregate's return value. If there is no final function then
the ending state value is returned as-is.

An aggregate function may provide an initial condition, that
is, an initial value for the internal state value. This is
specified and stored in the database as a value of type
text, but it must be a valid external
representation of a constant of the state value data type. If it
is not supplied then the state value starts out null.

If the state transition function is declared "strict", then it cannot be called with null
inputs. With such a transition function, aggregate execution
behaves as follows. Rows with any null input values are ignored
(the function is not called and the previous state value is
retained). If the initial state value is null, then at the first
row with all-nonnull input values, the first argument value
replaces the state value, and the transition function is invoked
at subsequent rows with all-nonnull input values. This is handy
for implementing aggregates like max. Note that this behavior is only available
when state_data_type is the same
as the first input_data_type.
When these types are different, you must supply a nonnull initial
condition or use a nonstrict transition function.

If the state transition function is not strict, then it will
be called unconditionally at each input row, and must deal with
null inputs and null transition values for itself. This allows
the aggregate author to have full control over the aggregate's
handling of null values.

If the final function is declared "strict", then it will not be called when the
ending state value is null; instead a null result will be
returned automatically. (Of course this is just the normal
behavior of strict functions.) In any case the final function has
the option of returning a null value. For example, the final
function for avg returns null when
it sees there were zero input rows.

Aggregates that behave like MIN
or MAX can sometimes be optimized
by looking into an index instead of scanning every input row. If
this aggregate can be so optimized, indicate it by specifying a
sort operator. The basic requirement is
that the aggregate must yield the first element in the sort
ordering induced by the operator; in other words

SELECT agg(col) FROM tab;

must be equivalent to

SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;

Further assumptions are that the aggregate ignores null
inputs, and that it delivers a null result if and only if there
were no non-null inputs. Ordinarily, a data type's < operator is the proper sort operator for
MIN, and > is the proper sort operator for MAX. Note that the optimization will never
actually take effect unless the specified operator is the
"less than" or "greater than" strategy member of a B-tree index
operator class.

Parameters

name

The name (optionally schema-qualified) of the aggregate
function to create.

input_data_type

An input data type on which this aggregate function
operates. To create a zero-argument aggregate function,
write * in place of the list of
input data types. (An example of such an aggregate is
count(*).)

base_type

In the old syntax for CREATE
AGGREGATE, the input data type is specified by a
basetype parameter rather than
being written next to the aggregate name. Note that this
syntax allows only one input parameter. To define a
zero-argument aggregate function, specify the basetype as "ANY"
(not *).

sfunc

The name of the state transition function to be called
for each input row. For an N-argument aggregate function, the
sfunc must take N+1 arguments, the first being of
type state_data_type and
the rest matching the declared input data type(s) of the
aggregate. The function must return a value of type
state_data_type. This
function takes the current state value and the current
input data value(s), and returns the next state value.

state_data_type

The data type for the aggregate's state value.

ffunc

The name of the final function called to compute the
aggregate's result after all input rows have been
traversed. The function must take a single argument of type
state_data_type. The return
data type of the aggregate is defined as the return type of
this function. If ffunc is
not specified, then the ending state value is used as the
aggregate's result, and the return type is state_data_type.

initial_condition

The initial setting for the state value. This must be a
string constant in the form accepted for the data type
state_data_type. If not
specified, the state value starts out null.

sort_operator

The associated sort operator for a MIN- or MAX-like aggregate. This is just an
operator name (possibly schema-qualified). The operator is
assumed to have the same input data types as the aggregate
(which must be a single-argument aggregate).

The parameters of CREATE AGGREGATE
can be written in any order, not just the order illustrated
above.